We continue to study the distribution of brain neurotransmitter or drug receptors by our newly developed autoradiographic method. Chemically coded neuronal pathways can be surmised from concordance of receptor distributions and autoradiographic tract tracing visualization. A novel radioimmunohistochemical method enables us to visualize neuropeptides and quantitate them simultaneously at any level of brain. Disappearance of neuropeptide following lesion of putative peptidergic tract provides proof of specific chemically coded pathways.